1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a funnel for a CRT to increase the BSN neck shadow margin by changing the shape of a yoke portion.
2. Description of the Related Art
FIG. 1 illustrates the configuration of a flat color cathode ray tube according to the related art.
Referring to FIG. 1, the flat color cathode ray tube is a kind of vacuum tube that includes a panel 1 which is a front glass and a funnel 2 which is a rear glass sealingly coupled with the panel 1 so that the flat color cathode ray tube is vacuum inside.
A fluorescent screen 4 is formed inside the panel 1. An electron gun 13 is installed at a neck portion of the funnel 2 opposed to the fluorescent screen 4.
A shadow mask 7 is installed between the fluorescent screen 4 and the electron gun 13, spaced by a predetermined distance from the fluorescent screen 4 to select colors. The shadow mask 7 is coupled with a mask frame 3, elastically supported by a mask spring 8 and supported on the panel 1 by a stud pin 12.
In addition, the mask frame 3 is coupled with a magnetic inner shield 9 to reduce the effect of the earth magnetic field in the rear of the cathode ray tube so that the movement of electron beam 6 caused by external magnetic field is reduced.
Meanwhile, a convergence purity correction magnet (CPM) 10 is installed at a neck portion of the funnel 2 to control an RGB electron beam so that an electron beam 6 emitted from an electron gun 13 is converged on one spot. The neck portion of the funnel 2 is provided with a deflection yoke 5 to deflect the electron beam and reinforcement band 11 to strengthen the front glass against internal vacuum.
The operation of the flat color cathode ray tube configured as described above will be described. The electron beam 6 emitted from the electron gun 13 is deflected in vertical and horizontal directions by the deflection yoke 5. The deflected electron beam 6 passes a beam passage hole of a shadow mask 7 and collides a front fluorescent screen 4 to display a predetermined desired color image.
For such a color cathode ray tube, consuming power is critical problem. So, the method of reducing the consuming power of the deflection yoke 5 has been studied.
Due to those studies, it has been developed that the outer diameter of a funnel yoke installation part on which a deflection yoke is installed is made to be small to reduce the space of deflection magnetic field so that the deflection field affects the electron beam efficiently.
However, if the outer diameter of the funnel yoke installation part is made only small, the electron beam deflected by the deflection yoke 5 collides the inner wall near to the neck of the funnel 2 to make the area A on the fluorescent screen 4 which the electron bean does not reach.
Accordingly, the conventional cathode ray tube is limited to reduce the outer diameter of the funnel yoke installation part.
In order to overcome this problem, in Japanese Laid-Open Patent publication No. 48-34349, it is described that the cross-sectional shape of the funnel is made to vary from circle to almost rectangle via ellipse as it goes from the neck portion to the funnel since the passage area along with a locus of the electron beam passing the yoke installation part in vicinity of the neck of the funnel is almost rectangular when drawing a raster that has a rectangular shape on the fluorescent screen.
In other words, referring to the cross-sectional shapes of in directions a—a, b—b, c—c, d—d and e—e of the funnel 1 and the funnel yoke installation part 14, it is found that the cross-sectional shape varies from circle to almost rectangle via ellipse as it travels from the neck portion to the funnel 1.
Accordingly, the deflection yoke has its cross-sectional shape is rectangular due to the shape of the yoke installation part of the funnel.
FIGS. 5a and 5b illustrate a deflection yoke that has rectangular cross-sectional shape.
Referring to FIGS. 5a and 5b, a deflection yoke includes a pair of horizontal deflection coils 21 for deflecting an electron beam emitted from an electron gun 13 in a horizontal direction, a pair of vertical deflection coils 22 for deflecting an electron beam emitted from an electron gun 13 in a vertical direction, a conical ferrite core 24 for reducing a loss of magnetic force generated by current passing through the horizontal deflection coils 21 and the vertical deflection coils 22 to enhance the efficiency of deflection, a holder 23 coupled directly with a neck portion of the funnel 2, a comma free coil 25 installed at the rear of the holder 23, for improving comma aperture, a ring band 25 installed at the rear of the holder 23, for coupling the funnel 2 with the deflection yoke 5 and a magnet 27 installed at outer side of an opening of the holder 23, for correcting raster distortion of screen.
The operation of the conventional deflection yoke 5 configured as described above will be described. The horizontal deflection coil 21 is provided with current that has a frequency of 15.75 KHz or more and deflects an electron beam in a horizontal direction using the magnetic field generated by the current. The vertical deflection coil 22 is provided with current that has a frequency of 60 Hz and deflects an electron beam in a vertical direction using the magnetic field generated by the current.
Generally, self-convergence method is employed to deflect electron beams by using non-uniform magnetic field to converge the electron beams-on a screen without any additional circuits or devices for each of three electron beams. The self-convergence method is the method to control the distribution of wires wound on the horizontal and vertical coils 21 and 22 and generate barrel or pin cushion magnetic field for each of the front portion, the middle portion and the rear portion of the deflection yoke 5 so that different deflection forces are applied to three electron beams 6 according to their locations to converge the electron beams 6 on the same point.
It is difficult that electron beams are deflected to the desired position by only the magnetic fields of the horizontal and vertical deflection coils 21 and 22. To compensate for this, a ferrite core 24 that has high permeability is used to minimize a loss of feedback path of the magnetic field to maximize the magnetic force.
FIGS. 6a and 6b illustrates combination of a funnel and a deflection yoke the cross-section of which is rectangle-shaped.
Referring to FIGS. 6a and 6b, in the configuration of the deflection yoke 5 as shown in FIG. 5a, 7n % or more of the deflection coils are distributed on a shorter side and a diagonal location of the deflection yoke 5, and mis-convergence correction ferrite sheet is attached to the upper portion of the deflection coils.
Accordingly, as shown in FIGS. 6a and 6b, referring to a diagram illustrating coupling of the deflection yoke 5 and a yoke installation part 14 formed at the funnel 2, the couple gap (FIG. 6b) between the shorter side (vertical side) of the deflection yoke 5 and the yoke installation part 14 is formed to be bigger than the couple gap (FIG. 6a) between the longer side (horizontal side) of the deflection yoke 5 and the yoke installation part 14.
The horizontal deflection coil 2 and a ferrite sheet are coupled in a gap between the shorter side of the deflection yoke 5 and the yoke installation part 14.
FIG. 7 is a perspective view of a yoke installation part coupled with a deflection yoke the cross-section of which is rectangular-shaped according to the related art.
Referring to FIG. 7, since the cross-section of the deflection yoke is rectangle-shaped, the cross-section of the yoke installation 14 of the funnel 2 is also rectangle-shaped.
As described above, since the cross-section of the deflection yoke is rectangle-shaped, the deflection sensitivity is improved and the consuming power is lowered. However, since the deflection sensitivity is improved and the movement of the electron beam gets very sensitive to the deflection yoke control, it causes a beam strike neck (BSN) phenomenon in which electron beam collides an inner wall of the yoke installation part 14 of the funnel so that the electron beam cannot reach a fluorescent screen.
When controlling yoke pull back (YPB) in installing a deflection yoke, the electron beam collides the yoke installation part to cause the BSN phenomenon in case that the deflection yoke is shifted to the neck of the funnel to optimize YPB.
To overcome this problem and ensure BNS neck shadow margin (NSM), employed are the method of shifting a deflection center of the deflection yoke to a tube axis and the method of enlarging the cross-sectional area perpendicular to the direction of the tube axis of the yoke installation part of the funnel to prevent the electron beam from colliding the funnel. However, these methods causes the side effect to deteriorate the deflection sensitivity of the deflection yoke and cause the deflection yoke to require more consuming power.
Accordingly, mainly used is the method of making the thickness of the portion of the funnel to be thin, which is collided by the electron beam. However, it lowers the productivity of the funnel to make the funnel thin, which is formed to have the minimal thickness.
In addition, it is disclosed in Korean Laid-Open Patent Publication No. 1998-25183 that the outer surface of the cross-section of the yoke installation part perpendicular to the direction of the tube axis is formed to non-circle-shaped and the inner surface of the cross-section is formed to be convex curved surface protruded toward the tube axis so that the vacuum exterior vessel is so strong to resist air pressure and the power for deflection is lowered. However, it has no effect to gain the BSN neck shadow margin.